Thermally stable heat transfer fluids and fluid systems

ABSTRACT

THE LONG TERM THERMAL STABILITY OF HALOGENEATED POLYPHENYL HEAT TRANSFER FLUIDS IS IMPROVED BY ADDING A PHOSPHOROUS COMPOUND TO THE FLUID AND PROVIDING AN INERT GAS PURGE TO SWEEP GASEOUS DECOMPOSITION PRODUCTS FROM THE HEAT TRANSFER SYSTEM. FLUIDS MAINTAINED IN THIS MANNER ARE LESS CORROSIVE, FORM LESS SOLID DECOMPOSITION PRODUCTS, AND UNDERGO A SMALLER VISCOSITY INCREASE THAN UNTREATED FLUIDS WHEN EXPOSED TO HIGH TEMPERATURE APPLICATIONS FOR EXTENDED PERIODS.

United States Patent Office 3,630,915 THERMALLY STABLE HEAT TRANSFERFLUIDS AN I) FLUID SYSTEMS James D. Sullivan, Webster Groves, Mo.,assignor to Monsanto Company, St. Louis, M0. N Drawing. Filed Oct. 27,1969, Ser. No. 869,918 Int. Cl. C09k 3/02 US. Cl. 25278 9 ClaimsABSTRACT OF THE DISCLOSURE The long term thermal stability ofhalogenated polyiphenyl heat transfer fluids is improved by adding aphosphorus compound to the fluid and providing an inert gas purge tosweep gaseous decomposition products from the heat transfer system.Fluids maintained in this manner are less corrosive, form less soliddecomposition products, and undergo a smaller viscosity increase thanuntreated fluids when exposed to high temperature applications forextended periods.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to heat transfer fluids and fluid systems wherein the thermaldecomposition of halogenated polyphenyl fluids is retarded by addingcertain phosphorous compounds to the fluid and providing an inert gaspurge to remove gaseous decomposition products from the system.

BACKGROUND OF THE INVENTION Halogenated polyphenyl compounds are wellknown as heat transfer fluids in the chemical industry and commonly usedwhere the fire resistance of the fluid is an important consideration.One class of fluids widely used in heat transfer systems are thechlorinated biphenyls. These fluids are fire resistant, have low vaporpressures as required for non-pressurized systems, and are inert andoxidation resistant at temperatures up to about 600 F.

When the chlorinated biphenyl heat transfer fluids are exposed totemperatures above 600 F., some polymerization occurs to form highboilers which are quite similar to the original material except for ahigher viscosity and boiling point. This polymerization representsthermal decomposition of the original fluid, and when a concentration ofapproximately to 10% of the high boilers have been formed, the fluidshows a sharp increase in viscosity.

Accompanying the formation of high boilers, small amounts of hydrogenchloride gas are evolved in the fluid. This gas then combines with themoisture present in the system to form acid and increase the corrosivityof the fluid. While many additives have been proposed as HCl scavengersto prevent the heat transfer fluid from becoming corrosive, the user ofthe fluid is still faced with the problem of fluid decomposition and theresulting increase in viscosity which has an adverse effect upon pumprequirements and fluid handling characteristics.

Heat transfer systems intended to be operated at temperatures in excessof 600 F. must now be designed with a side stream distillation facilityto constantly remove the higher boiling materials while the system is inoperation.

Fresh make-up fluid is periodically added to replace the quantity of thehigh boiler material removed. For smaller systems which are operated onan intermittent basis, it is common practice to completely drain thesystem and replace the fluid when the viscosity increases to apredetermined level. The used, high viscosity fluid is then reclaimed bydistillation.

The removal of high viscosity materials from these Patented Dec. 28,1971 heat transfer systems represents an inconvenience and an addedexpense in the operation of the system. A clear need is evident in theindustry for a method to prevent both viscosity increase and theformation of corrosive acids which normally result from thermaldecomposition of chlorinated biphenyl heat transfer fluids.

SUMMARY In the method of the present invention, thermal decomposition ofhalogenated polyphenyl heat transfer fluids such as chlorinated biphenylis retarded by incorporating an organic phosphite such as diisopropylphosphite in the fluid as a stabilizing additive, and by providing theheat transfer system with an inert gas purge such as nitrogen to removegaseous decomposition products. The thermal decomposition of the fluid,as measured by the rate of the viscosity increase, is drasticallyreduced to such an extent that the heat transfer system may be operatedfor extended periods at temperatures of 650 F. or higher with noappreciable viscosity change. Furthermore, the heat transfer fluidsmaintained in a system operated in accordance with the present inventionremain noncorrosive in both liquid and vapor phase, and have nosignificant amounts of solids accumulation over extended periods ofoperation.

It is, accordingly, an object of the present invention to provide atotal heat transfer system which overcomes the disadvantages of priorart systems by preventing or retarding the thermal decomposition of theheat transfer [fluids and the resulting increase in fluid viscosity,corrosivity, and solids or sludge accumulation.

DESCRIPTION OF PR'I-EFERRJ-ED EMBODIMENTS The essential components ofthe present invention are a heat transfer fluid, an organic phosphitestabilizer, and an inert gas purge. The hardware components of thesystem may be any of a multitude of present commercial designs, with theproviso that the hardware be adapted to accommodate the required inertgas purge.

The heat transfer fluids useful in the present invention include thepolyhalogenated polyphenyls, and particularly the chlorinated biphenyls,terphenyls and quaterphenyls. These compounds may contain from 30 tochlorine, and are commercially available. These fluids are well knownand widely used in heat transfer applications, although an uppertemperature limitation of about 600 F., is generally recommended toprevent thermal decomposition.

The organic phosphite additives useful in this invention are representedby the following structure wherein each R and R is individually selectedfrom the group consisting of straight chain and branched alkyls havingfrom 1 to about 10 carbon atoms including substituted alkyls and arylshaving from about 6 to 16 carbon atoms including alkyl, aryl, alkoxy,and aryloxy substituted aryls, and R can in addition be hydrogen.Specific examples of such organic phosphites are dimethyl phosphite,diethyl phosphite, diisopropyl phosphite, di-n-butyl phosphite,di-sec.-butyl phosphite, di-tert.-butyl phosphite, dipentyl phosphite,di-hexyl phosphite, di-n-propyl phosphite, dioctyl phosphite, trimethylphosphite, triethyl phosphite, tributyl phosphite, triphenyl phosphite,diphenyl phosphite, ditolyl phosphite, dixylyl phosphite,Z-ethyldiphenyl phosphite, 2,2-diethyldi-phenyl phosphite,3-chlorodiphenyl phosphite, 3-phenoxydipheny1 phosphite,4,4'-din-butyldiphenyl phosphite, phenyl di-n-propyl phosphite, phenyldi-n-butyl phosphite, phenyl di-sec.-butyl phosphite,

phenyl di-n-pentyl phosphite, phenyl dineopentyl phosphite, phenyldi-n-hexyl phosphite, p-methoxyphenyl din-butyl phosphite,m-chlorophenyl di-n-butyl phosphite, phenyl (n-propyl-n-pentyl)phosphite, phenyl (n-propyln-butyl) phosphite, phenyl (n-propyl-n-hexyl)phosphite, phenyl (n-butyl-n-pentyl) phosphite, phenyl (n-butyl-nhexyl)phosphite, phenyl (n-pentyl-n-hexyl) phosphite, phenyl(neopentyl-n-propyl) phosphite, phenyl (neopentyl-n-butyl) phosphite,phenyl (neopentyl-n-hexyl) phosphite, cresyl di-n-pentyl phosphite,tert.-butylphenyl di-n-butyl phosphite, n-butylphenyl di-n-butylphosphite, sec.-butylphenyl di-n-butyl phosphite, ethylphenyl di-nbutylphosphite, xylyl di-n-butyl phosphite, di(tridecyl) phosphite, triethylphosphite, tributyl phosphite, trioctyl phosphite, benzyl diethylphosphite, tricyclohexyl phosphite, tris-p-cyanoethyl phosphite,triphenyl phosphite, tri (3 phenoxyphenyl) phosphite, tri (3,5dimethylphenyl) phosphite, tri-(p-tert.-amylphenyl) phosphite andtris-(Z-chloroethyl) phosphite.

The amount of the organic phosphite added to the heat transfer fluid isadjusted in terms of the particular system, the fluid composition, andthe contemplated maximum operating temperature to which the fluid willbe exposed. In general, an amount of phosphite equal to from about 0.001to about 0.5% by Weight if the base fluid can be employed, although anamount from about 0.01 to about 0.05% is generally preferred for systemsoperating at 600 to 700 F. Amounts of additives greater than 0.5% can beused, but no commensurate advantages are obtained thereby.

Since such variables as operating temperature, hot spot temperature atheat transfer surfaces, system design, and ratio of metal surface areato fluid volume all influence the thermal stability of the fluid, theoptimum addition level is best determined individually for each system.

The inert gas purge is preferably applied to the heat transfer systemwherever the fluid is in contact with a vapor phase and provisions canbe made for venting the gas. Generally this is limited to the reservoiror surge tank since the remainder of the system is operated vaporfree.The inert gas should be sparged into the liquid for most effective useand most efficient removal of the gaseous decomposition products,notably HC], from the liquid. The sparge may be combined with an inertgas sweep across the surface of the fluid if desired.

In a preferred embodiment of this invention, inert gas is sparged intothe heat transfer fluid contained in the reservoir tank. A sparge rateof about 0.2 standard cubic feet of gas per hour per gallon of fluidbeing treated is inert gas generator is considered satisfactory for thisapplication.

The stabilizing effect of the combination of the organic phosphite andinert gas purge on the thermal decomposition of a heat transfer fluid isillustrated in the following examples. The metal corrosion data wereobtained by standard oxidation and corrosion tests conducted inaccordance with Federal Test Method Standards No. 791- Method 5308except for certain variations in fluid volume, test temperature, gascomposition, and metal material which are described in the followingprocedure.

A 100 gram sample of heat transfer fluid was placed in a sample tubewhich was approximately inches long and 2 inches in diameter. One irontest coupon was immersed in the fluid and another was suspended in thevapor space above the fluid. The sample tube was placed in a constanttemperature bath preheated to the desired test temperature. The inertgas purge, when included, was introduced through a tube inserted throughthe top of the sample tube and extending down into the sample to providea gas sparge below the surface of the fluid.

As the test proceeded, samples were withdrawn at regular intervals forviscosity determination. All viscosities were determined in an OswaldViscosimeter at 100 F. The change in viscosity with time was taken asthe measure of the thermal decomposition rate. Samples withdrawn forviscosity determination were returned to the sample tubes after themeasurement was completed.

The metal coupons were weighed and examined visually to determine theeffect of the fluid and fluid vapors on the metal. In the followingexamples, a weight gain of less than about 0.1% was not consideredsignificant. Larger increases in weight were due to the formation ofsolids either from the deposit of excess additive or from the corrosionof the iron. Weight loss was attributed to metal loss due to corrosion.In most cases, the appearance of the sample was the most significantmeasure of the cf fectiveness of the system in preventing metal attack.

Example 1 A heat transfer fluid consisting of chlorinated biphenylcontaining 48% chlorine was evaluated in a series of tests involvingdifferent diisopropyl phosphite additive levels and nitrogen purgerates. Table I below summarizes the fluid compositions and the resultsof the evaluation in terms of viscosity increase, iron attack, formationof sludge, and appearance of the iron coupon and the fluid after test.The test was conducted at 650 F. for a period of 168 hours.

Weight gain in mg. per sq. cm. of iron surface.

generally adequate. The quantity of fluid being treated is It isapparent from the above data that there is no sigthat amount of fluidcontained in the tank above the nificant difference in the resultsobtained between the point of sparge. The optimum sparge rate may varyfrom about 0.05 to about 1.0 standard cubic feet per hour per gallon offluid being treated, depending upon the operating conditions of thesystem, the volume of fluid contained in the system, and the volume ofthe reservoir tank. The optimum level of gas sparge is, therefore, bestdetermined individually for each system.

The sparged gas may be any inert gas composition, with nitrogen, carbondioxide, or mixture thereof being upper and lower levels of additiveconcentrations and nitrogen purge rates used in this example. It is alsoseen that the inert purge alone significantly stabilizes the fluidagainst viscosity increase, but fails to prevent rusting of the ironcoupon and discoloration of the fluid. Similarly, the use of theadditive without the inert gas purge essentially prevents iron attack,but fails to prevent a change in fluid viscosity. Only by using thecombination of the additive and the inert gas purge can the system bestapreferred for reasons of economy. Moist gas from an bilized againstboth viscosity increase and iron attack.

6 Example 2 of the phosphite and the nitrogen purge. Exp. 3 withExperiment 3 of Example 1 was repeated substituting phosphite alonetermmateq after 600 hours While 2 carbon dioxide for the nitrogen inertgas purge. Similar i i mtrogen alone tefmuiated atfer T results wereobtained, with a viscosity increase of 4.8% It ls Seen.that thecombmatlpn of the pho.sphlt.e and i and an iron attack value of 0.09 mg.per square cm. of gen Provldes more than twlc? the operating Me of ironsurface. The test demonstrates that carbon dioxide gen alone and near.1yt1mes.that of the .ptiosphlte may be substituted for nitrogen as thepurge material withalone clearly ther? IS an i q i or synerglsilc effectout effect on the heat transfer System. between the organic phosphiteaddltlve and the lnert gas sweep which is not evldent until after 100hours of opera- EXflmPh? 3 10 tion at temperatures in excess of 600 F.,and which effect The heat transfer fluid and diisopropyl phosphiteaddihas heretofore been unknowntive of Example 1 were evaluated in along-term test for Example 4 1000 hours at 650 F. Iron coupons weresubjected to the action of the liquid and the vapor during this testperiod. In a commercial size heat transfer system, a surge tank At theconclusion of the test period, the coupons were 5 approximately 10 feethigh and 4 feet in diameter and weighed to determine weight loss orgain, and inspected normally containing from about 150 to 200 gal. ofheat for appearance and the presence of any coatings or detransfer fluidwas equipped with an inert gas sparge conposits. The percent viscosityincrease of the fluid was also sistin of a pipe formed into a circularring and having determined as a measure of the extent of thermaldecominch diameter holes drilled on 1 inch centers around positionoccurring during the extended test period. The 20 the circumference ofthe ring. The ring was positioned betest data obtained are presented inTable II below. low the normal minimum liquid level in the tank. DuringTABLE II operation of the system, fluid was circulated through the 1000hm at 6500K tank at a rate of from about 60 to 300 gal. per min., andinert gas was sparged into the fluid at a rate of about 0.2 Additive N2Percent 1111 attack Appearance cu. ft. per hr. per gal. of fluidcontained in the tank above wt. purge, viscosity In In iron the point ofsparge. Sparged gas was vented from the top percent L/hr' Defense hqmdcoupons of the tank. The inert gas sparge rate of about 0.2 cu. ft. 10.02 0.25 100 +0.11 +0.13 Iridescent. per hr. per gal. of fluid beingtreated was found to be a 2 0 100 46 47 3555;, near optimum level forthis system. In general, a sparge 2 0. 03 g b i i8 g tg g g rate of atleast 0.5 and preferably from about 0.1 to about 0.5 cu. ft. per hr. pergal. of fluid under treatment is used. figs; 288 831:- At lower rates,the volume flow of gas may be insufficient to sweep the gaseousdecomposition products from the The data from the 1000 hour testconfirmed the results system, while higher rates provide no additionalbenefit obtained in Example 1. Specifically, it is evident that both andare economically unjustified. the additive and the purge are required tostabilize the The preceding examples serve to illustrate the practicefluid against viscosity increase, and to reduce the amount of thepresent invention, but the invention is not intended of iron attack inthe formation of coatings on the iron to be limited thereto. Forexample, given the knowledge samples. It is evident from Experiment 2that a small ni- 40 of this invention, it is within the ability of oneskilled trogen purge, while stabilizing the viscosity of the fluid, inthe art to modify the described procedure without dedoes not afford anyprotection to the iron exposed to the parting from the spirit or scopeof the present invention. liquid or the vapor. From Experiment 3, it isseen that For example, the concentration of the stabilizing addiwhilethe additive reduces the corrosivity of the fluid, it tive or the rateof the inert gas flow may be varied, or the contributes very little toviscosity stabilizationinert gas may be simultaneously sparged into andswept Experiments 1 and 2 were continued beyond the 1000 across theliquid heat transfer fluid. Heat transfer systems hour test period tostudy the change in fluid viscosity and having multiple reservoirs orother vapor spaces may emdetermine maximum fluid life. Viscosity datafor these ploy various combinations of inert gas sparges and two samplesare presented in Table III below. sweeps. Furthermore, it iscontemplated that the method TABLE III Percent Viscosity Increase at 650F.

Time, hrs.

Experimentl 12 13 27 35 so 145 344 460 670 1,400 Experiment 2 30 54 125275 1,608 Experiment3 1,500

The data in Table III above clearly illustrate the fluid of the presentinvention may be combined with the constability in three possible heattransfer systems operating tinuous purge and recovery systems of theprior art. All

for extended periods of time at 650 F. Exp. 1 is typical such variationsin equipment and procedure are contemof the systems of this inventionwhich include both the plated to be within the scope of the presentinvention. nitrogen purge and the phosphite additive. Exp. 2 was run Theembodiments of this invention in which an excluunder identicalconditions but with the nitrogen purge sive property or privilege isclaimed are defined as folonly, and Exp. 3 was run with the additiveonly. lows:

Heretofore, test data up to 1000 hours exposure time indicated that thenitrogen purge was primarily responsi- 1. In a heat transfer systemutilizing halogenated biphenyl, halogenated terphenyl, halogenatedquaterphenyl,

ble for preventing fluid viscosity increase, and that the ador mixturesthereof as a heat transfer fluid between a heat dition of the phosphiteadditive had little absolute effect source and a heat sink, theimprovement comprising: in this regard. It was indeed surprisingtherefore, that (A) incorporating in the heat transfer fluid from aboutwhen Exp. 2 was terminated after 2500 hours with a visp f PX Weight ofthe fluid of all cosity increase of about 1600 percent, the fluidviscosity in ol'ganlc P P StablllZef represented y the Exp. 1 hadincreased only 80 percent. mula It is apparent from the data in TableIII that the difference between Exp. 1 and Exp. 2 is far greater thancould reasonably be attributed to a simple additive effect wherein eachR and R is individually selected from the group consisting of alkylradicals having from 1 to about carbon atoms and aryl radicals havingfrom about 6 to 16 carobn atoms, and provided that R can in addition behydrogen, and,

(B) sparging an inert gas into the heat transfer fluid at a rate fromabout 0.05 to about 1.0 standard cubic feet per hour per gallon of heattransfer fluid and purging from the system the inert gas together withgaseous thermal decomposition products.

2. A heat transfer system of claim 1 wherein the aryl radicals areselected from the group consisting of alkyl, aryl, alkoxy, and aryloxysubstituted aryls.

3. A heat transfer system of claim 1 wherein the heat transfer fluid isa chlorinated biphenyl.

4. The system of claim 1 wherein the organic phosphite stabilizer isdiisopropyl phosphite.

5. A heat transfer system of claim 1 wherein the inert gas is selectedfrom the grou consisting of nitrogen, carbon dioxide, and mixturesthereof.

6. In a heat transfer system utilizing a halogenated biphenyl,halogenated terphenyl, halogenated quaterphenyl, or mixtures thereof asa heat transfer fluid between a heat source and a heat sink andincluding a fiuid reservoir tank having a vapor space above a quantityof fluid, the improvement comprising:

(A) incorporating in the heat transfer fluid from about 0.01 to about0.05% by weight of an organic phosphite compound represented by theformula ROPOR 6 H wherein each R is individually selected from the groupconsisting of alkyl radicals having from 1 to about 10 carbon atoms and(B) sparging into the fluid contained in the reservoir tank asubstantially continuous stream of inert gas at an average rate of fromabout 0.05 to about 1.0 standard cubic feet per hour per gallon of fluidcontained in the tank above the point of sparge and purging from thesystem the inert gas together with gaseous thermal decompositionproducts.

7. A method for stabilizing the viscosity of halogenated polyphcnyl heattransfer fluids selected from the group consisting of halogenatedbiphenyl, halogenated terphenyl, halogenated quaterphenyl and mixturesthereof which comprises sparging a stream of inert gas into the fluid atan average rate of from about 0.05 to 1.0 standard cubic feet per hourper gallon of heat transfer fluid and purging from the system the inertgas together with gaseous thermal decomposition products.

8. A method of claim 7 wherein the halogenated biphenyl is chlorinatedbiphenyl having from about 30% to 60% by weight chlorine.

9. A method of claim 7 wherein the inert gas is selected from the groupconsisting of nitrogen, carbon dioxide and mixtures thereof.

References Cited UNITED STATES PATENTS 2,136,774 11/1938 Hickman 532,741,598 4/1956 Good 25278 3,115,465 12/1963 Orlolf et al 252-78 X3,150,516 9/1964 Linnenbom et al. 5553 X 3,280,031 10/1966 Brennan etal. 25249.8 3,496,107 2/1970 Lima et al. 252499 LEON D. ROSDOL, PrimaryExaminer D. SILVERSTEIN, Assistant Examiner

